From WO 03/013950 in the name of the applicant, an anchor line installation method is known according to which a pre-installed anchor line carrying a connector at its free end, is retrieved by the installation vessel. The connector may be formed by a chain stopper so that it functions as a chain tensioner device. A pulling line attached to a winch on the installation vessel is guided along a sheave on the chain tensioner device on board of the installation vessel. The installation vessel also picks up a free pendant end that is connected to the floating structure and the pulling line is connected to this free pendant end. The chain tensioner device is over boarded and is connected below water level with the pendant by hauling in the pulling wire on the winch.
By placing the chain tensioner device externally from the moored structure, in line with the anchor leg, the installation and commissioning of the tensioning system is removed from the construction yard schedule of the moored structure. Also, a reduction in chain table size and removal of installation aids from the turret is achieved in case the chain tensioner is used in turret-moored offshore structures. For spread-moored FPSO's, utilising the known external in-line tensioner results in space saving at the topside due to the absence of tensioner devices such as chain jacks and chain stoppers and the absence of storage space for excess installation chain. Furthermore, no maintenance is required on chain jacks on the moored offshore structure and during re-tensioning operations no disruption of the production and processing operations occurs.
The known method has as a disadvantage that the pendant that is connected to the floating structure has a relatively long length, which makes it difficult to store on and to deploy it from the floating structure. Furthermore, checking for a proper connection between the known chain tensioner device and the pendant requires underwater inspection, such as by divers. If for some reason a proper connection between the pendant and the tensioner device is not made, the connection operation needs to be interrupted.
From U.S. Pat. No. 7,059,262 a mooring line connection method is known wherein a pre-installed mooring line with at its free end a clamping device is placed on the sea-bed.
The clamping device comprises a slotted sheave or pulley and a chain stopper that is attached to a recovery pendant and a submersed buoy. The buoy is attached to a pull in wire on an anchor-handling vessel via a ROV and the clamping device is hoisted on board of the handling vessel. On board of the vessel the pendant from the floating structure, having at its free end a chain part, is pulled through the clamping device, and is attached to the pulling wire. Next, the clamping device is over boarded and the mooring line is pulled upward such that the pendant chain part moves along the chain stopper of the clamping device that functions as a ratchet allowing only one directional movement of the chain, until the right tension in the mooring line is achieved.
In the known method, the connection between the chain tensioner and the chain part of the pendant connected to the floating structure is made below water level, which results in difficult control over the connection procedure. Also abandoning the clamping device on the seabed may cause ingress of sand or small stones into the clamping device resulting in malfunctioning or blocking of the tensioner device. Again, the relatively long length of the pendant connected to the floating structure in this mooring line installation method results in relatively difficult handling.